1. Field of the Invention
The present invention relates to a free fall stroker for raising and releasing a drop hammer attached to a cable, and in particular to a compact, hydraulically driven, horizontally movable pulley assembly for engaging, raising and releasing a cable attached to the hammer for precisely controlling the hammer movement in a soil testing apparatus.
2. Description Of the Related Art
An established procedure for testing soils density and composition involves taking a core sample every five feet for analysis. To accomplish this, first a drilling rig drills a five foot deep hole using a hollow bore auger. A core sampler, which comprises a pipe section, which may be 16", 24" or 30" long and which has a 2" outside diameter and a 1.5" inside diameter and an attached 140 lb. drop hammer which is dropped 30". The density of the soil being tested is determined based upon the number of drop strokes required to drive the sampler a set distance into the soil. In order to operate within the drilled holes, the weights used generally take the form of a cylindrical drop hammer. For accurate testing, the drop length must be precisely controlled.
Typical prior art drop hammers include a hammer head attached to a cable which is reeled onto a winch spool. Once the hammer is raised to the desired height, the spool is released, thus dropping the hammer.
In such systems, a number of problems can occur. For example, the cable can bind as it is wound onto the winch spool, which prevents the hammer from dropping when the spool is released. Furthermore, winch release mechanisms are not always reliable, can inadvertently trip, and tend to slow the descent of the hammer due to friction within the spool itself. While these problems are not necessarily critical in a drop hammer used for pile driving or chiseling, for example, they are completely unacceptable in a soil testing apparatus which requires precise control.
It is clear then, that an improved apparatus and method for raising and releasing a drop hammer is needed. Such an apparatus should be compact and inexpensive, extremely reliable and rugged, should permit the drop hammer to drop in a virtual free fall, with little or no retarding action due to friction within the apparatus, and should allow repeated identical hammer drops to achieve precise soil bearing capacity measurements.